Session_End

[sunburnt]

When is the Session_End() event fired?
I knocked out this code...

        protected void Session_End(Object sender, EventArgs e)
        {
            FileStream fStream = new FileStream("C:\\out.txt",FileMode.Create);
            ASCIIEncoding ascEnc = new ASCIIEncoding();
            fStream.Write(ascEnc.GetBytes("end event."),0,"end event.".Length);
            fStream.Close();
        } 

And something similar in the Session_Start event, but this one never fired after I navigated away from the page. Am I missing something?